1. Field of the Invention
The present invention relates to a zoom lens system having a high vari-focal ratio, and more specifically a zoom lens system which has an unlimited back focal length and is suited for use as a photographic lens system for compact cameras (35 mm compact cameras in particular).
2. Description of the Prior Art
As zoom lens systems which have a simplest composition, there are conventionally known zoom lens system each consisting of two lens units.
The zoom lens systems each consisting of the two lens unit provide merits such as a merit to permit simplifying a structure of a lens barrel, but have a defect to allow aberrations to be varied remarkably, thereby making it impossible to enhance vari-focal ratios.
As a conventional example of the zoom lens systems each consisting of two lens units, there is known a zoom lens system disclosed by Japanese Patent Kokai Publication No. He 9-152549 which uses a second lens unit consisting of two positive and negative lens components and has a vari-focal ratio close to 3. In order to obtain a zoom lens system which is more compact than the conventional zoom lens system or has a vari-focal ratio higher than that of the conventional zoom lens system, however, it is necessary to strengthen refractive power of a second lens unit, whereby aberrations are produced in larger amounts and varied more remarkably by changing a magnification.
Furthermore, each of zoom lens systems disclosed by Japanese Patents Kokai Publication No. Sho 64-52111 and No. Hei 8-110468 which are other conventional examples uses a second lens unit which consists of three lens components to share negative refractive power among the lens components and lower curvature on lens surfaces, thereby reducing amounts of aberrations to be produced and variations of the aberrations to be caused by changing a magnification.
When the refractive power of the second lens unit is strengthened, refractive power of each lens component is strengthened correspondingly, whereby a variation of an airspace in the lens unit produces a large influence on a change of a focal length.
When a second lens unit is composed of a negative lens component, a positive lens component and a negative lens component as in a zoom lens system disclosed by Japanese Patent Kokai Publication No. Hei 8-304701, Hei 9-127415 or Hei 10-148756, it is possible to cancel an influence due to the positive lens component with those due to the negative lens components, thereby preventing a change of a location of a focal point.
A primary object of the present invention is to provide a zoom lens system which comprises, in order from the object side, a first lens unit having positive refractive power and a second lens unit having negative refractive power, and is configured to change a magnification by varying an airspace between the first lens unit and the second lens unit, wherein the first lens unit comprises, in order from the object side, at least a first subunit which has negative refractive power and a second subunit which is composed of a lens component or a cemented lens component consisting of two or more lens elements and has positive refractive power, wherein the second lens unit comprises a first negative lens component, a second positive lens component and a third negative lens component which are disposed in order from the object side with airspaces interposed, and wherein all lens components disposed in the first subunit having the negative refractive power of the first lens unit satisfy the following condition (1):
xe2x88x921 less than fG1/fLi less than 0.3xe2x80x83xe2x80x83(1)
Another object of the present invention is to provide a zoom lens system which comprises, in-order from the object side, a first lens unit having positive refractive power and a second lens unit having negative refractive power, and is configured to change a magnification at a ratio of 2 or higher by varying an airspace between the first lens unit having the positive refractive power and the second lens unit having the negative refractive power, wherein the second lens unit having the negative refractive power comprises a negative lens component, a positive lens component and a negative lens component which are arranged in order from the object side with airspaces interposed and formed by molding a resin material, and satisfies the following condition (3):
xe2x88x920.03(1/mm) less than MGL/|f21| less than 0.03(1/mm)xe2x80x83xe2x80x83(3)
Still another object of the present invention is to provide a zoom lens system which comprises, in order from the object side, a first lens unit having positive refractive power and a second lens unit having negative refractive power, and is configured to change a magnification by varying an airspace between the first lens unit having the positive refractive power and the second lens unit having the negative refractive power, characterized in that the second lens unit having the negative refractive power comprises a first negative lens component, a second positive lens component and a third negative lens component which are disposed in order from the object side with airspaces interposed, and that the positive lens component has an Abbe""s number of 40 or larger.